Scholarship at UWindsor
Scholarship at UWindsor
Electronic Theses and Dissertations Theses, Dissertations, and Major Papers
1-1-1970
An alphanumeric text generator for a computer display.
An alphanumeric text generator for a computer display.
William E. Mennie University of Windsor
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Mennie, William E., "An alphanumeric text generator for a computer display." (1970). Electronic Theses and Dissertations. 6621.
https://scholar.uwindsor.ca/etd/6621
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ProQuest Information and Learning
300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 800-521-0600
AN ALPHANUMERIC TEXT GENERATOR
FOR A COMPUTER DISPLAY
BY
WILLIAM E. MENNIE
A T hesis
Submitted to th e F a cu lty o f Graduate S tu d ies through the Department o f E l e c t r i c a l E n gin eering i n P a r t ia l F u lfillm e n t
o f the Requirements fo r the Degree o f Master o f Applied S cien ce a t th e
U n iv e r sity o f Windsor
Windsor, Ontario
1970
®
UMI
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APPROVED
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2 1 8 1
2
This t h e s is d e sc r ib e s th e l o g i c a l d e sig n and. im plem entation o f th ree
v a r ia tio n s o f an alpha-num eric t e x t gen erator fo r d isp la y in g computer
inform ation on a cathode ray tube ( c . r . t . ) .
Each ch aracter i s generated by in t e n s if y in g d o ts which are s e le c t e d
from a 5 x 7 m a trix . S e le c t io n o f th e d o ts i s performed by d i g i t a l l o
g ic c i r c u i t s which su p p ly unblanking p u lse s to th e Z -a x is o f th e c . r . t .
i n synchronism w ith a r a s t e r g en era to r.
Three methods o f r a s t e r g en era tio n are d e sc r ib e d . This perm its the
ch a ra cter generator to be u sed i n system s o f v a ry in g c o s t and co m p lex ity .
The most complex method u se s h o r iz o n ta l and v e r t i c a l d i g i t a l to
analogue (d/a) co n v erters to generate th e r a s t e r as w e ll as t o p o s it io n
each ch a ra cter. This perm its s e v e r a l new ch a ra cter o p era tio n s to be
provided by hardware.
Three s i z e s o f ch a ra cters may be generated and two s i z e s may be
hardware p o s itio n e d as s u b s c r ip ts o r s u p e r s c r ip t s . Back sp acin g en a b les
s u p e r sc r ip ts to be p o s itio n e d above s u b s c r ip ts o r ch a ra cters to be super
im posed. T abulation lo c a t e s up to e ig h t v e r t i c a l columns. C arriage r e
tu rn s may be made to any one o f n in e le ft - h a n d margin p o s it io n s . Line
fe e d s may be s in g le or double sp aced . Spacing between l i n e s and char
a c te r s i s a u to m a tic a lly adjusted, accordin g to th e ch aracter s i z e u sed .
C ontrol o f th e ch a ra cter gen erator may be accom plished through an
ASR 33 T eletype and a few fu n c tio n k e y s. Thus th e t e x t generator i s
e a s ie r to op erate than most co n v e n tio n a l ch a ra cter g en era to rs.
i i
The t e x t gen erator fu n c tio n s e q u a lly w e l l w ith a sto ra g e c . r . t . or
w ith a r e fr e sh memory s in c e i t i s capable o f g en era tin g up to 53»000
ch a ra cters per second .
The s i m p li c it y and v e r s a t i l i t y o f the t e x t g en erator makes i t
s u it a b le fo r n e a r ly e v ery a p p lic a t io n which u t i l i s e s a c . r . t . d is p la y
s in c e i t can r e a d ily be t a i l o r e d to such u s e r ’s needs as the a d d itio n
o f a graphics c a p a b il it y .
xax
The author w ishes to ex p ress h is a p p r e c ia tio n to Dr. P . A. V.
Thomas fo r s u g g e stin g t h i s p r o j e c t and fo r h is h e lp f u l c r it ic i s m .
Acknowledgement must a ls o go t o th e N a tio n a l R esearch C ouncil o f Canada
fo r the f in a n c ia l a s s is t a n c e which made t h i s p r o je c t p o s s ib le .
i v
TABLE OF CONTENTS
Page
ABSTRACT ... . . . i i
ACKNOWLEDGEMENTS ... . . . i v
TABLE OF CONTENTS . . . ... v
LIST OF ILLUSTRATIONS... v i i i LIST OF TABLES . ... x i I . INTRODUCTION 1 .1 The D e s ir a b il it y o f Cathode Ray Tube D isp la y s * . 1 1 .2 Some Current Methods o f C haracter G eneration • • 4 1 . Beam Shaping 4 2 . Scanning . . . 4
3> Stroke or Waveform . . . 3
4 . Dot Generation ... 5
5* Software Techniques . . . . . 6
1 .3 The R efresh Problem . . . 7
1 .4 Three Proposed Methods o f C haracter G eneration . 8 1 .5 Im plem entation . . . 9
I I . GENERATION OF UNIQUE CHARACTER PULSE TRAINS 2 .1 Reasons fo r S e le c t in g the 5 x 7 Dot M atrix . . . 10
2 .2 The Unique P ulse Train . . . 12
2 .3 General D e sc rip tio n o f th e C haracter Dot Generator 12 2 .4 The C o u n te r ... 1^
2 .5 The Counter Decoder . . . 17
2 .6 P u lse S e le c t io n Gates . . . . . . 20
v
2 .8 The C haracter Decoder ... 24
I I I . TWO METHODS OF BEAM POSITIONING USING AN ANALOGUE HORIZONTAL SWEEP 3*1 In tr o d u ctio n ... 26
3 .2 H o rizo n ta l and. V e r t ic a l Sweep Method . . . 26
3 .3 C haracter Generator w ith H o r iz o n ta l Sweep and V e r t ic a l D/A C onverter ... . . . 34
IV . BEAM POSITIONING BY USING D/A CONVERTERS 4 .1 In tr o d u ctio n % 4 . 2 One P o s s ib le Method ... . . . 38
4 .3 The General Method Used 39 4 . 4 C haracter S iz e S e le c t io n 42 4 .5 C haracter Count C on trol C ir c u itr y ... 4?
4 .6 X R e g is te r Count S e l e c t i o n . 49
4 .7 The X R e g i s t e r ... 51
4 .8 Y R e g iste r Count S e le c t io n . . . 54
4 .9 Y R e se t C ir c u itr y . . . ... . . . 58
4 .1 0 The Y R e g is te r . . . . 61
4 .1 1 UP-DOWN Count S e le c t io n C ir c u itr y . . . 61
V. SPECIAL CHARACTERS 5 .1 In tr o d u c tio n . . . • 64
5 .2 S p e c ia l C haracter Decoder . . . 64
5 .3 The Back Space . . . 68
5*4 The Line Feed. 68
v i
Page
5 .5 The C arriage Return and Tabulate . . . 69
1 . C arriage Return . . . . . . 69
2 . Tabulate . . . . 73
VI. CONTROL AND TESTING OF TEE A/N GENERATOR 6 .1 R efresh and. C ontrol o f th e a/N D isp la y . . . 79
6 .2 The
i / o
T e r m in a l... 826 . 3 T est R e su lts . . . . . 86
V II. CONCLUSIONS ... 88
APPENDIX I CHARACTER LEST AND ASCII CODES... 92
APPENDIX I I CHARACTER DOT PATTERNS AND PULSE NUMBERS . . . . 9^
APPENDIX I I I CHARACTER SIZE AND POSITION EXAMPLES.... 100
APPENDIX IV INSTRUCTION AND DATA WORDS... 101
APPENDIX V TIMING DIAGRAMS ... 102
APPENDIX VI LOGIC NOTATION . 105
APPENDIX VII TWO OPERATING SPEEDS FOR HORIZONTAL AND VERTICAL SWEEP CHAR AC'TER GENERATION... 106
REFERENCES ... 107
VITA AUCTORIS... 108
v i i
Figure Page
1 .1 Examples o f C haracters Generated by D iff e r e n t Methods 3
2.1 Use o f th e Unique Pulse Train . . . 11
a . H o rizo n ta l D e fle c tio n V oltage Waveform
b . V e r t ic a l D e fle c tio n Waveform
c . P u lse Train Required to Form th e Character 'A'
d . C haracter Which R e su lts From Combined Waveforms
2 .2 Block Diagram o f C haracter Dot Generator . . . 13
2 .3 Counter fo r H o rizo n ta l Sweep Method . ... 15
2 . ^ High Speed Counter fo r H o rizo n ta l d/a Converter Method 16
2 .5 The Counter Decoder 18
2 .6 a The B a sic 5 x 7 Dot M atrix . . . 19
2 .6 b The P u lse S e le c t io n Gates 19
2 .7 The B u ffer R e g is te r ... 21
2 .8 The C haracter Decoder ... 23
3 .1 Output o f O r ig in a l Character Generator ... 2?
3 .2 Character Generator With H o rizo n ta l and V e r tic a l Sweep 29
3 .3 H o rizo n ta l Sweep and V e r tic a l D/A Converter Method . 32
3 .4 Timing Waveforms fo r H o rizo n ta l Sweep and V e r tic a l D/A Converter . . . 33
a . H orizo n ta l D e fle c tio n V oltage Waveform
b . Clock P u lses
c . Y -R eset P u lses to C lear the L east S ig n ific a n t Three B its
Figure Page
d . Count Up In th e Y D ir e c tio n
e . V e r t ic a l D e fle c tio n Waveform
4 .1 One P o s s ib le Method Using d/a C o n v e r t e r s ... . 37
4 .2 a Normal Counter ... 40
4 .2 b Counter Capable o f Counting i n Any P o s itio n ... 40
4 .3 Siae S e le c t io n C ir c u itr y ... 43
4 .4 a Dot M atrix . . . ... . . . 46
4 .4 b C haracter Count C ontrol C ir c u itr y . . . ... 46
4 .5 X Count S e le c t io n C ir c u itr y ... 50
4 .6 a X R e g is te r (L ea st S ig n if ic a n t B i t s ) . . . 52
4 .6 b X R e g is te r (Most S ig n if ic a n t B i t s ) . . . 53
4 .7 Y Count S e le c t io n C i r c u i t r y ... ... 56
4 .8 Y R e se t C ir c u itr y ... • 57
4 .9 a Y R e g is te r (L east S ig n if ic a n t B i t s ) . . . 59
4 .9 b Y R e g is te r (Most S ig n if ic a n t B i t s ) ... 60
4 .1 0 UP-DOWN Count S e le c t io n C ir c u itr y . . . 62
5 .1 S p e c ia l C haracter D e c o d e r ... 65
5 .2 a G eneration o f th e Line Feed P u l s e ... 67
5.2 b Use Margin R e g iste r and Space F lip -F lo p . . . 67
5 .3 X R e g is te r C lear C ir c u itr y . . . 70
5 .4 C ir c u itr y fo r Loading X R e g is te r During C arriage Return and. Tabulate ... 71
5 .5 Tabulate Algorithm ... 75
5 .6 C ir c u itr y Required To Generate 6 4 's Count During Tab u la t io n . . . . . . . 76
6 .1 a D evice S e le c to r Schematic ... 80
i x
6 .1 b a/N C on trol U n i t ... 80
6 .2 Block Diagram o f i / o Terminal . . . 81
6 .3 Three R a ster Techniques (Photographs) . ... 84
a . H o rizo n ta l and V e r t ic a l Sweep
b . H o rizo n ta l Sweep and V e r t ic a l D/a Converter
c . H o rizo n ta l and. V e r t ic a l D/A Converter
6 .4 a Three C haracter S iz e s (P h o to g r a p h )... 85
6 .4 b Tabulate and C arriage Return ( P h o t o g r a p h ) ... 85
x
LIST OF TABLES
Table Page
4 .1 X R e g iste r Counting C a p a b ilitie s ... 49
4 .2 Y R e g iste r Counting C a p a b ilitie s 54
4 .3 Y-Reset P o s itio n s . . . 55
5 .1 Tabulate and Margin P o s itio n s 69
x i
INTRODUCTION
1 .1 The D e s i r a b i l i t y o f Cathode Ray Tube D isp la y s
D ig it a l computers are capable o f ex trem ely ra p id com putation and
data m an ip u lation . S in ce m echanical d e v ic e s such as ty p e w r ite r s , l i n e
-p r in t e r s , card rea d ers and -p l o t t e r s are g e n e r a lly much slow er than th e
computer, th e computer must norm ally slow down to match th e speed o f the
in p u t-o u tp u t
( i / o )
d e v ic e s which are tr a n s m ittin g data to and r e c e iv in gdata from th e computer. This can vary from a person ty p in g a few char
a c te r s per minute to a l i n e p r in t e r capable o f o u tp u ttin g perhaps 3»000
ch a ra cters per secon d .
To avoid t h i s problem, modern h ig h -sp eed computers g e n e r a lly tr a n s
f e r th e computed in fo rm a tio n in t o b u ffe r s which can a c c e p t data a t the
com puter's h igh speed and th en su p p ly t h i s data to th e output d e v ic e a t
i t s own lower sp eed . The r ev e r se o p era tio n can be used fo r lo a d in g data
in t o th e computer. Thus th e computer i s a b le to in t e r a c t w ith s e v e r a l
in p u t-o u tp u t d e v ic e s to ensure th a t th e h ig h computing c a p a b ilit y i s
used to the b e s t advantage. For th e m a jo rity o f s c i e n t i f i c and b u sin ess
a p p lic a tio n s t h i s type o f in t e r a c t io n i s s a t i s f a c t o r y . The job i s sub
m itted a t one tim e and th e computed r e s u lt s are o b ta in ed on paper a t a
l a t e r tim e.
More r e c e n t ly , computers have found t h e ir way in t o such v a r ie d areas
13
as data r e t r i e v a l , com puter-aided in s t r u c t io n (CAI) and com puter-aided
d e sig n (CAD). These u se s in tro d u ce two new problems which the above
1
2
mentioned
i / o
d e v ic e s are n o t r e a d ily a b le to cope w ith . F i r s t , theu ser i s in t e r e s t e d i n ra p id r e s u lt s and cannot a ffo r d lo n g d e la y s w h ile
w a itin g fo r p r in te d m a t e r ia l. O b viously, he cannot s i t i n f r o n t o f a
l in e - p r in t e r a l l day. S econ d ly, th e u se r o ft e n w ish es to i n t e r a c t w ith
th e computer. How i s he a b le to o b ta in a c ce ss to th e in fo rm a tio n which
has been computed b u t i s s t i l l i n th e output b u ffe r and n o t y e t p r in ted
so th a t he may r a p id ly d ecid e i f th e r e s u l t s are what he expected ?
The b e s t s o lu t io n to th e problem developed so fa r i s th e use o f a
cathode ray tube ( c . r . t . ) d is p la y . C haracter w r itin g speeds i n e x c e ss
o f 50,000 per second are e a s i l y a c h ie v ed . Thus computed data can be
tra n sm itted and d is p la y e d alm ost in s ta n ta n e o u s ly . The a d d itio n o f a
lig h t - p e n and fu n c tio n keys to th e normal ty p ew r ite r keyboard, adds th e
p o s s i b i l i t y o f m odifying or p o in tin g to s e le c t e d a rea s o f th e d is p la y
to o b ta in th e a t t e n t io n o f th e computer fo r ra p id two-way in t e r a c t io n
between th e computer and th e u s e r . Adding a grap hics c a p a b il it y en
a b le s p i c t o r i a l as w e l l a s p r in te d in fo rm a tio n to be d is p la y e d .
Another im portant advantage o f c . r . t . d is p la y s i s th e p o s s i b i l i t y
o f having s e v e r a l c . r . t . scr e en s d is p la y in g th e same in fo rm a tio n sim
u lt a n e o u s ly . This i s advantageous, fo r example, i n modern a ir p o r ts
where computers are used i n f l i g h t sch ed u lin g . D isp la y s p laced i n
s t r a t e g i c lo c a t io n s throughout the term in al can im m ed iately inform the
s t a f f and p u b lic o f a r r iv a l and departure tim e s.
The fe a tu r e s mentioned above make c . r . t . d is p la y s an a t t r a c t iv e
and e f f i c i e n t method o f p ro v id in g communication between the computer
and i t s u s e r s . I n a d d itio n , th ey can be made e s t h e t i c a l l y p le a s in g to
th e eye and t h e i r q u ie t o p e r a tio n perm its th e u se r to work i n a p e a c e fu l
and th o u g h t-stim u la tin g environm ent.
B
(a ) Beam Shaping
(b ) Scanning
B B
( c ) Stroke or Waveform# • • • .
•
•
•
•
• • • •
•
•
•
•
• • • •
(d ) DotFigure 1.1 Examples o f C haracters Generated by D iffe r e n t Methods
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2 7 1 .2 Some Current Methods o f C haracter G eneration '
There are many d if f e r e n t methods o f gen era tin g ch a ra cters fo r c . r . t .
d is p la y s i n use tod ay. The most im portant requirem ent i s th a t th ey are
e a s i l y read ab le. A few o f the c u r r e n tly used methods w i l l be d e sc r ib e d .
1 . Beam Shaping
This method u t i l i z e s a s p e c i a l l y c o n str u c ted c . r . t . w ith a
s t e n c i l mask p laced i n th e path o f th e e le c t r o n beam. This s t e n c i l con
t a in s th e shape o f a l l ch a ra cters which can be d is p la y e d . The unshaped
e l e c t i o n beam generated by the a c c e le r a t in g gun o f th e tube can be de
f l e c t e d to a p a r tic u la r area o f the mask by s e l e c t i o n p la t e s w ith in the
tu b e . The beam p a sses through the mask and i s extruded i n the shape o f
the ch a r a cter . The shaped beam i s then d e f le c t e d a g a in to i t s f i n a l
p o s it io n on the s c r e e n . Although t h i s method o f ch a r a cter g en eration
i s f a s t , the tube i s ex p en siv e and th e number and shape o f the charac
t e r s i s u s u a lly q u ite lim it e d . F ig . 1 .1 a i s an example o f t h i s type o f
c h a r a c te r .
2 . Scanning
This method u t i l i z e s a scanning r a s t e r s im ila r to th a t used
i n t e l e v i s i o n . The ch a ra cter data w ith in th e g en erator i s scanned in
synchronism w ith th e r a s t e r to provide th e data r eq u ir ed to modulate
the beam. This method i s w e ll s u it e d to m u ltip le d is p la y s such as the
p r e v io u sly mentioned a ir p o r t d is p la y s in c e t e l e v i s i o n s e t s can be used
and in terco n n ected by c o a x ia l ca b le fo r a r e l a t i v e l y low c o s t d is p la y .
This method cannot r e a d ily be used fo r l i g h t pen in t e r a c t io n s in c e the
e n t ir e screen i s b eing scanned by th e r a s t e r and th e co o rd in a tes o f
the beam a t a p a r tic u la r time are n o t a c c u r a te ly known. F ig . 1 .1 b g iv e s
an example o f t h i s c h a r a c te r .
3 . Stroke o r Waveform
T his method o f g en era tin g c h a r a cter s probably perm its the
g r e a t e s t f l e x i b i l i t y o f ch a ra cter s t y l e s . The ch a ra cter i s sto r e d as
X d e f l e c t i o n , Y d e f l e c t i o n and i n t e n s i t y in fo rm a tio n . As the ch aracter
i s b ein g gen era ted , th e se th ree p ie c e s o f analogue d ata are sim u ltan e
o u s ly fe d to the X, Y and Z axes o f th e c . r . t . tu b e . The ch aracter i s ,
t h e r e fo r e , tr a c ed ou t i n much th e same manner as one would w r ite char
a c te r s by hand. This method i s v e ry f a s t bu t th e c i r c u i t r y req uired to
s to r e and su p p ly th e analogue in fo rm a tio n i s q u ite complex and e x p e n siv e.
F ig . 1 .1 c i l l u s t r a t e s th e ch a ra cters generated by t h i s method.
4 . Dot G eneration
I n t h i s method, th e ch a ra cters are composed o f a group o f
d o ts which are s e le c t e d from a b a s ic a rra y or m a tr ix . Two op era tio n s
must be performed to gen erate a ch a r a cter by t h i s method. F i r s t , the
d o t m atrix p o s it io n must be generated on th e scr e en by e it h e r a sm all
sweep r a s t e r or by d/a c o n v erters which can a c c u r a te ly p o s itio n th e beam
w ith in the a rra y . S econ d ly, some means o f d eterm ining whether to i n
t e n s i f y (unblank) th e beam a t a p a r tic u la r p o s it io n w ith in th e array
must be provided fo r each c h a r a c te r .
One o f th e most commonly used methods o f g en era tin g the i n
t e n s i f i c a t i o n in fo rm a tio n i s to u se a sm a ll core memory. I f a 5 x 7
m atrix o f d o ts i s u sed , th e memory w i l l c o n ta in 35 m agnetic co res a ls o
arranged i n a 5 x 7 m a trix . A sen se w inding i s threaded through th e
co res which correspond to th e d o ts which are req u ired to generate a
p a r tic u la r c h a r a c te r . I n a d d itio n , a s e t and r e s e t w inding i s threaded
6
through each c o r e . A ll o f th e s e t w indings o f each column are connec
te d i n s e r ie s as are th e r e s e t w indings o f each row.
To generate a ch a ra cter th e core memory i s in te r r o g a te d i n
sequence w ith the r a s t e r g e n e r a tio n . F i r s t , a l l o f th e cores o f a par
t ic u l a r column are s e t bu t th e ch a ra cter sen se w indings are d is a b le d .
Then each core i n th e column i s r e s e t i n sequence and th e sense winding
i s a c tiv a te d . Only i f the sen se w inding p asses through a core which i s
bein g r e s e t w i l l i t sen se th e changing magnetic f i e l d and c o n tr o l th e
in t e n s i f i c a t i o n o f th e corresponding p o in t i n th e c . r . t . r a s t e r . The
obvious d isadvantages o f t h i s method are th a t ch a ra cters cannot be changed
w ithout rew irin g th e core m atrix and a complete read c y c le must be per
formed fo r a l l 35 p o s s ib le d o ts i n th e m atrix making th e system q u ite
slow compared w ith o th er methods.
Other methods o f s e l e c t i n g d o ts have a ls o been u sed . One u ses
a diode m atrix which i s scanned to generate th e x and y d e f le c t io n as
w e ll as th e req uired i n t e n s i t y in fo rm a tio n .
5 . Software Techniques
In order to gen erate ch a ra cters as r a p id ly or as in e x p e n s iv e ly
as p o s s ib le , the v a rio u s hardware tech n iq u es which have been mentioned
have u s u a lly been employed to generate c h a r a c te r s. U su a lly o n ly the
type and s i z e o f th e c h a ra cter i s su p p lie d to the ch aracter gen erator.
The a c tu a l gen era tio n o f th e ch a ra cter i s accom plished a u to m a tic a lly .
With th e advent o f ex trem ely high speed random a c ce ss memories,
some manufacturers are moving away from hardware character g en era tio n .
The coordinate and i n t e n s i t y in form ation req u ired to generate a p a r tic u
l a r character i s s to r e d i n a memory as a subroutine which can e a s i l y be
m od ified t o generate any shape and s i z e o f ch a ra cter one p le a s e s ,
A ch a ra cter i s generated by s e l e c t i n g a p a r tic u la r su b ro u tin e,
The s to r e d in form ation i s then fe d to d /a co n v erters which c o n tr o l th e
beam p o s itio n in g and i n t e n s i t y whenever t h a t p a r tic u la r subroutine i s
ad d ressed. The obvious advantages o f su ch a method are f l e x i b i l i t y and
th e use o f r e l a t i v e l y u n so p h is tic a te d hardware. But u n t i l r e c e n t ly h ig h
speed memories have been v ery c o s t l y alth ou gh t h e ir p r ic e i s r a p id ly be
in g reduced now. I t may w e ll be th a t i n th e v ery near fu tu re t h i s method
o f ch a ra cter gen era tio n may overtak e th e c o n v en tio n a l hardware te c h n iq u e s,
1 .3 The R efresh Problem
I t has been im p lied above th a t h ig h speed i s a d e s ir a b le ch a ra cter
i s t i c fo r a ch a ra cter g en era to r. This can be d e s ir a b le fo r two main r ea
so n s .
F i r s t , i t was m entioned t h a t one w ish es to have th e computer tr a n s
f e r i t s computed r e s u lt s as r a p id ly as p o s s i b l e . I f one i s u t i l i z i n g a
s p e c ia l sto r a g e c . r . t . tube w hich i s capable o f s to r in g th e image tr a c ed
o u t by the e le c tr o n beam, th e computer can tr a n s fe r th e d ata to the d i s
p la y a t a f a i r l y h ig h speed (s a y 3»000 c h a r a c te r s/se c o n d ) and th e scr e en
w i l l r e t a in th e in fo rm a tio n . This type o f d is p la y cannot be u sed fo r
in t e r a c t iv e d is p la y s which u se a l i g h t pen s in c e th e d is p la y i s con
tin u ou s w ith tim e and does n o t g iv e the computer any means o f determ in
in g the p o s it io n o f the pen.
I f one i s not u t i l i z i n g a sto ra g e c . r . t . tu b e , g en era tin g the d i s
p la y once i s n o t s u f f i c i e n t s in c e i t fad es r a p id ly w ith tin ® . This
fa d e -o u t or p e r s is te n c e tim e t y p i c a l l y v a r ie s from between 20 t o 100
m illis e c o n d s depending on th e type o f phosphor used i n th e c . r . t . This
b rin g s us to th e second and m ost im p ortant need fo r a h ig h sp eed d is p la y .
To m ain tain th e in fo rm a tio n on a n o n -s to r in g c . r . t . , th e d is p la y
must c o n tin u a lly be reg en era ted o r r e fr e s h e d . This r e fr e s h in g must be
done a t a r a te which i s h ig h enough to preven t th e eye from s e e in g i t
o r th e scr e en w i l l appear to have annoying f l i c k e r . Thus th e r e f r e s h
r a te cou ld be as h ig h as 50 o r 60 frames per second depending on the
phosphor o f th e c . r . t . and background l i g h t i n g o f the room. In order
to d is p la y 1 ,0 0 0 c h a ra cters on th e s c r e e n , one must have a ch a ra cter
gen erator capable o f g e n e ra tin g 5 0 ,0 0 0 ch a r a cter s per secon d . Hence
so ftw a re c h a ra cter g e n e r a tio n i s c u r r e n t ly v e ry lim it e d due to the
amounts o f in fo rm a tio n r e q u ir e d .
1 .4 Three Proposed Methods o f C haracter G eneration
A 5 x 7 d o t m a trix was chosen to provide th e ch a ra cter r a s t e r .
The method o f s e l e c t i n g and i n t e n s i f y i n g th e c o r r e c t d o ts f o r a p a r t i
c u la r c h a ra cter i s common to a l l th r e e ch a ra cter g e n e r a to r s. This w i l l
be d is c u s s e d i n g r e a te r d e t a i l i n chapter I I .
The th re e c h a ra cter gen era to rs d i f f e r p r im a r ily i n th e method used
to generate th e 5 x 7 d o t m a trix . The f i r s t method u t i l i z e s both h o r i
z o n ta l and v e r t i c a l sweep c i r c u i t s to generate th e ch a r a cter r a s t e r . A
d/a co n v erter i s a ls o used i n th e v e r t i c a l a x is i f more th an one l i n e o f
ch a ra cters i s to be d is p la y e d .
The second method u t i l i z e s a h o r iz o n ta l sweep c i r c u i t but o n ly a
D/a co n v erter i s used i n th e v e r t i c a l a x is to generate b oth th e seven
v e r t i c a l d o t p o s it io n s i n th e m atrix and th e p o s itio n in g o f s e v e r a l l i n e s
o f t e x t .
The f i n a l method, u se s a d/a co n v e rter fo r th e h o r iz o n ta l p o s it io n
in g as w e ll as a v e r t i c a l d/a co n v erter s im ila r to t h a t used i n the s e c
ond method.
The th ree methods o f g en era tin g the ch aracter r a s te r s have been
co n stru cted and t e s t e d . They w i l l be d isc u sse d i n d e t a i l i n chapters
TTT and IV along w ith t h e ir advantages and d isa d v a n ta g es.
10
1 .5 Implementation
The c i r c u i t r y which w i l l be d e scr ib e d i n th e remainder o f t h i s
t h e s i s was co n stru cted from D i g i t a l Equipment C orporation (DEC) ’’FLIP
CHIP" m odules. This i s mentioned a t t h i s tim e, s in c e some o f th e c i r
-c u it r y whi-ch i s to be d is -c u s se d i s v e r y dependent on DEC hardware.
However, when t h i s i s the c a s e , enough th eo ry i s g iv en so th a t one could
e a s i l y implement the c i r c u i t u s in g o th e r c i r c u i t r y .
The m a jo rity o f the l o g i c was implemented u sin g DEC 'R' s e r i e s lo g i c
modules which can operate up t o a frequency o f 2 MHz. These modules use
n e g a tiv e "NAMD" lo g i c i n w hich 0 v o l t s rep resen ts a "zero" or " f a ls e ”
s t a t e and -3 v o l t s rep resen ts a "one" or "true" s t a t e .
The d/a co n v erters were DEC 'A' s e r i e s m odules. A few 'B* s e r ie s
d e la y modules were used s in c e th ey provide fo r d ela y s as sh o r t as 50 n se c .
s e r ie s clamped load s were used whenever a d d itio n a l s ig n a l clamping
was req u ired .
At t h i s p o in t, i t i s su g g e ste d t h a t the reader fa m ilia r iz e h im se lf
w ith th e symbols and lo g ic n o ta tio n d escr ib e d i n Appendix VI.
CHAPTER I I
GENERATION OF UNIQUE CHARACTER PULSE TRAINS
2 .1 Reasons fo r S e le c t in g th e 5 x 7 Dot M atrix
At an e a r ly sta g e i n th e developm ent o f th e ch a ra cter g en era to r,
i t was d ecid ed t h a t a T eletyp e Model 33 ASR (Automatic S end-R eceive)
would be used to gen erate th e ASCII ch a r a cter codes which would th en be
fe d to the d is p la y v ia th e computer o r com p u ter-refresh system . Thus
o n ly upper ease ( c a p it a l) l e t t e r s would be g en era ted . I t i s p o s s ib le
t o generate a l l o f th e p r in t in g ch a r a cter s found on th e keyboard w ith
a 5 x 7 d o t m atrix.
I f one had w ished to g en erate low er ca se c h a r a c te r s, a t l e a s t a
7 x 9 m atrix would be n e c essa ry t o provide s u f f i c i e n t d o t r e s o lu t io n
fo r l e g i b l e and d i s t i n c t c h a r a c te r s. But th e a d d itio n o f o n ly two d o ts
t o th e h o r iz o n ta l and v e r t i c a l axes o f th e m atrix means t h a t a t o t a l o f
6 3 d o t p o s itio n s would be req u ired in s te a d o f 35* This means t h a t ap
p roxim ately 75$ more hardware would be req u ired j u s t to provide th e bas
i c d o t m atrix g en era tio n n o t to m ention a d d itio n a l hardware req u ired to
s e l e c t th e g r ea ter number o f d o ts which c o n s t it u t e each c h a r a cter .
I n a d d itio n to in c r e a se d hardware, speed becomes an im portant fa c to r
when a la r g e r d o t m atrix i s u se d . S in ce each d o t i s generated sequen
t i a l l y i n tim e, i t would req u ir e a 75% in c r e a s e i n th e ch a ra cter genera
t in g tim e fo r a 7 x 9 m atrix i f th e same d o t frequency were u sed . Hence
i f a r e f r e s h system were to be u sed , th e number o f ch a ra cters which could
10
(a) H o rizo n ta l D e f le c t io n V oltage Waveform
t^ = tim e req u ired to gen erate 35 p u lse s
(ch a ra cter tim e)
t g = tim e req u ired to gen erate 14 p u lse s (ch a r a c ter sp a cin g )
Time (b ) V e r t ic a l D e fle c tio n Waveform
( c ) P u lse Train Required to Form the Character 'A'
©
• •
(d ) C haracter Which R e su lts From Combined Waveforms
Figure 2 .1 Use o f th e Unique P u lse Train
12
be d isp la y ed would be n ea rly c u t i n h a lf !
I f one were to generate a 7 x 9 d o t m atrix a t a h igh er speed, th e
c o s t o f th e lo g i c c i r c u i t r y would be in c r e a s e d . In a d d itio n to t h i s ,
m odulation o f the Z -a x is ( i n t e n s i t y ) o f th e c . r . t . becomes d i f f i c u l t a t
h ig h fr e q u e n c ie s. Since one w ish es to generate d o ts and n o t smeared
_7
l i n e s , i n t e n s i f i c a t i o n tim es must be much l e s s than 100 n sec (10 s e c
onds) fo r the d e s ir e d ch aracter g en era tin g sp eed . This fa c to r would
n e e d le s s ly add to th e c o s t o f th e d is p la y c . r . t .
2 .2 The Unique P u lse Train
As th e 35 p o s itio n s o f th e 5 x ? m atrix are b ein g generated sequen
t i a l l y w ith tim e, in form ation must be fe d to th e c . r . t . i n t e n s i f i c a t i o n
c i r c u i t s t o determ ine whether a p a r tic u la r d o t i s to be i n t e n s i f i e d or
n o t. F ig . 2 .1 i l l u s t r a t e s t h i s fo r th e sim ple ca se o f a h o r iz o n ta l and
v e r t i c a l sweep r a s t e r . As th e r a s t e r i s b ein g generated s e q u e n tia lly
i n tim e, the unique p u lse t r a in fo r th e l e t t e r A i s being su p p lied to
th e Z a x i s . The r e s u lt in g ch a ra cter i s shown i n F ig . 2 . I d . Thus some
method o f gen era tin g a unique p u lse t r a in i s req u ired fo r a l l p r in tin g
characters bein g generated by th e d is p la y . This w i l l be d isc u sse d i n
the remainder o f t h i s ch ap ter.
2 .3 General D e sc r ip tio n o f the C haracter Dot Generator
The a c tu a l gen era tio n o f th e unique p u lse t r a in s fo r each ch aracter
i s common to a l l th re e ch aracter generators which are d isc u sse d i n the
remainder o f t h i s t h e s i s . Only th e c o n tr o l u n it and the r a s te r generator
vary i n th e th ree system s and one s l i g h t m o d ific a tio n must be made to a
cou n ter.
The ch aracter d o t generator i t s e l f c o n s is t s o f a c lo c k , a s i x b i t
p ro d u c e d w ith p e rm is sio n of the c o p y ri g h t o w n e r. F u rth e r re p ro d u c tio n p ro h ib ite d w ith o u t pe rm is s io n .
SPECIAL
CONTROL CHARACTER€>,
P CHARACTER
BUFFER
UNIT DECODER DECODER PARALLEL DATA CHANNEL PULSE COUNTER SELECTION DECODER GATESz - S f r s
TO HORIZONTAL AND VERTICAL RASTER TIMING DELAY
CLOCK COUNTER
14-co u n ter, a 14-counter d e14-cod er, g a tin g c i r c u i t r y to s e l e c t unique p u lse tr a in s
fo r each character and a b u ffe r to h o ld th e ASCII code o f th e ch a ra cter
whose p u lse t r a in i s to be g ated to th e Z -a x is . In a d d itio n , an i n h i b i t
g a te i s used to p rev en t th e p u lse t r a in from reachin g th e Z -a x is i n sp e
c i a l c a s e s . A b lo ck diagram o f th e ch a ra cter d o t gen erator i s g iv e n i n
P ig . 2 . 2 .
2 .4 The Counter
A s i x b i t b in a ry up co u n ter which i s capable o f cou n tin g up to
2^ = 6 4 i s u sed . T h ir ty -F iv e counts correspond to th e 35 d o ts i n the
5 x 7 m a trix . However, space must be in s e r t e d between c h a r a c te r s.
I f a sweep c i r c u i t i s used to provide th e h o r iz o n ta l d e f le c t io n ,
th e beam i s moving a t a c o n sta n t r a t e . To provide two h o r iz o n ta l r a s t e r
u n it s (d o t p o s it io n s ) o f sp a cin g between c h a r a c te r s, enough tim e must
pass to provide f o r an a d d itio n a l 14 d o ts ( i . e . two columns o f 7 d ots
e a c h ). Hence 49 c lo c k p u lse s must be counted. This type o f coun ter i s
used by the r a s t e r g en era to rs which w i l l be d is c u s se d i n Chapter H I .
I f th e h o r iz o n ta l beam p o s it io n i s produced by a b in ary cou n ter and
a D/A co n v e rter , o n ly one a d d itio n a l p u lse a f t e r th e 35 m atrix counts
w i l l be req uired to provide th e s p a c e . This w i l l be d is c u s se d i n d e t a i l
i n Chapter IV.
A NAND gate d e t e c t s a coun ter v a lu e o f 35 fo r th e D/A co n v erter
method or a count o f 48 fo r th e h o r iz o n ta l sweep method. The outpu t o f
t h i s gate en a b les th e DCD g a te o f a p u lse a m p lifie r . Thus th e 36th or
th e 49th c lo c k p u lse p a sse s through th e p u lse a m p lifie r . This p u lse i s
used to c le a r th e co u n ter , th e b u ffe r and th e "GENERATE" f l i p - f l o p which
i s a d ev ice busy f l a g i n th e c o n tr o l u n i t . When t h i s f l i p - f l o p i s s e t ,
CLOCK GENERATE BUFFER
AND GENERATE
Figure 2 .3 C ou n ter.for H orizontal Sweep Method
e p ro d u ce d w ith p e rm is sio n of the c o p y ri g h t o w n e r. F u rth e r re p ro d u c tio n p ro h ib ite d w it h o u t per m is s io n .
CLEAR A/N BUFFER, GENERATE SPACE, CHARACTER COMPLETE
GENERATE
CLOCK
i t en a b les th e cou nter; c le a r in g i t d is a b le s th e c o u n ter . The 36th c lo c k
p u lse a ls o g en era tes th e c h a ra cter space i n th e c a se o f th e d /a co n v erter
method. P ig . 2 .3 i l l u s t r a t e s the cou nter which would be used w ith a
h o r iz o n ta l sweep c i r c u i t . I t i s capable o f o p e r a tin g a t a c lo c k frequency
o f 1 .0 8 MHz.
One s e r io u s problem which arose from u sin g DEC 2 me modules was
t h a t each f l i p - f l o p has a nom inal ca rry prop agation o f 70 n s e c . This
means th a t one must w a it fo r up to 420 n sec fo r th e cou n ter to reach a
ste a d y s t a t e . But a t 2 MHz, the c lo c k p u ls e s are 500 n sec a p a rt. Hence
o n ly 80 n sec i s a v a ila b le during which i n t e n s i f i c a t i o n w ith i t s a s s o c i
a ted g a tin g propagation d e la y s can take p la c e . This i s i n s u f f i c i e n t .
To e lim in a te ca rry tim e a DC ca rry ch a in was u se d . This perm its
th e c o n str u c tio n o f a cou n ter o f any s i z e w ith a l l f l i p - f l o p s sw itch in g
sim u lta n e o u sly . I t c o n s is t s o f s i x in tercon n ected , diod e g a te s w ith tw o,
t h r e e , . . . , s e v e n in p u ts each o f which has an ou tp u t o n ly when the in p u t
to t h a t gate and t o a l l th e g a te s o f l e s s e r s ig n if i c a n c e are i n th e one
s t a t e ( i . e . a ca rry w i l l occu r on th e n e x t c o u n t). I n t h i s way th e i n
put DCD g a tes o f th e f l i p - f l o p s through which a ca rry would propagate
are en a b led . Sin ce th e c lo c k i s fe d to a l l f l i p - f l o p s i n p a r a l l e l , th ose
w ith enabled DCD g a te s w i l l be complimented sim u lta n e o u sly . Hence no
propagation d e la y w i l l occur fo r a c a rr y . F ig . 2 . 4 i l l u s t r a t e s a 2 MHz
cou n ter which would be used fo r th e d/a c o n v erter method.
2 .5 The Counter Decoder
A sequence o f 35 p u lse s i s req u ired to i n t e n s i f y each d o t o f the
5 x 7 m a trix . A s e r i e s o f 35# 7 -in p u t NAND g a te s i s u sed i n the counter
decod er. S ix o f th e in p u ts are connected to th e outpu ts o f th e s i x
18
COUNTER BIT 1
HO)
8(0) 16( 0 ) 3 2 ( 0 )
PULSE
;>To Pulse S e le c t io n Gates
COUNTER BIT 1 (1 ) " 2 (1 )
PULSE
^>To Pulse S e le c t io n Gates 16(0 )
3 2 (1 ) #35
DELATED CLOCK
Figure 2 .5 The Counter Decoder
7 # 1 4 # 21 # 2 8 # 3 5 #
6 9 1 3 9 2 0 # 2 7# 3 4 #
5 # 1 2 # 1 9 # 2 6 # 3 3 #
4 # 1 1 # 1 8 # 2 5 # 3 2 #
3 # 10 # 1 7 # 2 4 # 3 1 #
2 # 9 # 1 6 # 2 3 # 3 0 #
1 # 8 # 1 5 # 2 2 # 2 9 #
(a ) The B a sie 5 x 7 Dot Matrix
PULSE NUMBER 1
A1 S e le c t io n Gate 24
PULSE NUMBER 4
12
'<—* Selec-jC t i o n Gate / 20
Figure 2 .6 b The Pu lse S e le c t io n Gates
To *A'
_► ^
C haracter Decoder Gate
To
-* >
C haracter Decoder Gate
20
f l i p - f l o p s o f th e co u n ter i n such a way th a t the f i r s t g a te d e t e c t s a
count o f one, th e secon d a count o f two, up to th e t h i r t y - f i f t h which
d e te c ts a count o f 35*
The sev en th in p u t to each o f th e g a tes i s a d ela y ed c lo c k p u ls e .
R efer to f i g . 2 .5 » The c lo c k must be d elayed because th e c lo c k p u lse s
are f i r s t used to change th e co u n ter . A fte r the cou nter i s changed by
the c lo c k p u lse and th e 35 g a te s have reached a s te a d y s t a t e c o n d itio n ,
the delayed c lo c k p u lse reaches th e g a t e s . This p rev en ts the am biguity
which cou ld occur i f th e 35 decoder g a te s were sampled a t th e same time
the counter was changing.
Thus each NAND g a te w i l l have an ou tp u t when i t s p a r tic u la r count
i s p resen t i n th e c o u n te r . S in ce o n ly one gate can be enabled a t any
one tim e, th e g a te s provide 35 d i s t i n c t p u lse s which are s e q u e n tia l i n
tim e.
2 .6 P ulse S e le c t io n Gates
Each ch a r a cter which i s to be generated i s composed o f unique d o ts
s e le c t e d from the b a s ic 5 x 7 m atrix which i s shown i n f i g . 2 .6 a . As
the va rio u s d o t p o s it io n s are generated by th e r a s t e r g en erator (sweep
or d/a c o n v e r te r ), a d e c is io n as to whether to i n t e n s i f y a d o t or n o t
must be made f o r each p a r tic u la r c h a r a c te r . This i s done by th e p u lse
s e l e c t i o n NAND g a t e s .
The in p u ts to th e s e g a te s are the outputs o f th e counter decoder
NAND g a te s . I f a c h a r a c te r r e q u ir e s d o t #1 o f th e m a trix , the output
o f th e f i r s t cou n ter decoder NAND gate i s u sed . Hence th e p u lse s e l
e c t io n gate fo r a p a r t ic u la r ch a ra cter w i l l have as many in p u ts as th ere
are d o ts i n th e com pleted c h a r a c te r . S in ce th e in p u ts were ou tp uts o f
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the preceding counter decoder NAND g a te s the fo llo w in g Boolean e x p r e ss
io n w i l l be true i f p u lse s 8 f 9 , 15» 16 were to be u s e d
:-PULSE TRAIN = :-PULSE #8 A :-PULSE #9 A :-PULSE #15 A :-PULSE #16
= PULSE #8 V PULSE #9 V PULSE #15 V PULSE #16
Thus we s e e th a t th e p u lse s e l e c t o r NAND g a te s a c t as OR g a t e s . The
output o f th ese g a te s w i l l be a unique p u lse t r a in fo r each ch a ra cter
to be generated.
At p resen t a s i x t y ch a r a cter s e t o f "printing" ch a ra cters i s used
bu t t h i s could be expanded to any s i z e by the a d d itio n o f more p u lse
s e l e c t o r NAND g a te s . A l i s t o f a l l the p u lse s req u ired to form each
ch a ra cter and a model o f each ch a ra cter can be found i n Appendix I I .
Samples o f th e g a tes are shown i n F ig . 2 .6 b .
2 .7 The B uffer R e g is te r
This r e g is t e r h o ld s th e ASCII code fo r th e ch a ra cter which i s cur
r e n t ly b eing gen erated . Although th e ASCII code r eq u ir es 8 b i t s to d e fin e
th e f u l l ch aracter s e t , n o t a l l b i t s need be u t i l i z e d . I f one o n ly w ish es
to generate the p r in tin g c h a r a cter s o f the ASR 33 T elety p e , th e s i x l e a s t
s i g n i f i c a n t b i t s o f th e code would be s u f f i c i e n t s in c e 2^ = 64 ch a ra cters
could be s e le c t e d .
I n our c a se , seven b i t s were used so th a t n o n -p rin tin g o p era tio n s
such as l i n e fe e d , c a r r ia g e r e tu r n , ta b u la te and back space co u ld be d e
r iv e d d i r e c t l y from th e t e le t y p e keyboard. This a ls o provid es f o r ex
pansion o f up to 2? = 128 c h a r a c te r s. A com plete l i s t o f th e ASCII codes
used w i l l be found i n Appendix I .
The b u ffe r which h o ld s th e ASCII code c o n s is t s o f seven f l i p - f l o p s
which can be loaded i n p a r a l l e l . The c o n tr o l o f th e lo a d in g o p era tio n
BUFFER BIT 7(1
" 2 (0 ) OUTPUT OF 'A' SELECTION GATE
A cts As An 'OR'
Gate o f C.R.T
BUFFER BIT 7(1
" 2( 1 ) OUTPUT OF V-' SELECTION GATE
Figure 2 .8 The C haracter Decoder
2k
would be by computer or by a s p e c ia l c o n tr o l u n i t . This w i l l be d i s
cussed fu rth er i n the fo llo w in g c h a p te r s. The d is p la y b u ffe r r e g is t e r
(DBR) could be the accumulator o f th e computer o r th e memory b u ffe r
r e g is t e r o f a r e fr e s h memory. F ig . 2 .7 shows th e b u ffe r in p u ts as be
in g the seven l e a s t s i g n i f i c a n t b i t s o f th e r e fr e s h b u ffe r which w i l l
be described, b r i e f l y i n chapter V I.
2 .8 The Character Decoder
The reader has probably noted t h a t th e unique p u lse t r a in fo r ev
e r y ch aracter was generated sim u lta n eo u sly by th e P ulse S e le c t io n G ates.
Some means o f s e l e c t i n g o n ly th e ch a ra cter p u lse t r a in whose ASCII code
i s contain ed i n the b u ffe r must be p rovid ed . This s e l e c t i o n i s per
formed by th e character decoder.
At p r e se n t, s i x t y p r in tin g ch a ra cters are generated and th ey o n ly
req u ire s i x b a ts o f the b u ffe r to d e fin e them u n iq u e ly . Thus th e se v
en th b i t i s n o t needed a t t h i s p o in t and w i l l be d is c u s se d fu r th e r i n
th e chapter on S p e c ia l C h aracters.
The ch a ra cter decoder i s comprised o f a s e t o f s i x t y 7 -in p u t NAND
g a tes w ith a g a te corresponding to each c h a r a c te r . The f a c t th a t o n ly
s i x b i t s d e fin e the s i x t y ch a ra cters provid es a s u b s t a n t ia l sa v in g s in
hardware s in c e one l e s s in p u t i s needed fo r each gate (an 8 -in p u t NAND
gate would have been req u ired i f a l l seven b u ffe r b i t s were u s e d ). S ix
o f th e in p u ts are fe d by th e output o f the s i x l e a s t s i g n i f i c a n t b u ffer
f l i p - f l o p s . Each decodes a d if f e r e n t ASCII c o d e. The sev en th in p u t i s
fe d by the ou tp ut o f th e P u lse S e le c t io n gate which g en era tes th e p u lse
t r a in fo r th e ch aracter s p e c if ie d by th e ASCII code on th e o th er s i x
in p u ts . The ch aracter decoder i s i l l u s t r a t e d i n F ig . 2 . 8 . The fo llo w
in g Boolean e x p r e s s io n a p p l i e s
DECODER OUTPUT = PULSE TRAIN A ASCII CODE
or DECODER OUTPUT = PULSE TRAIN A ASCII CODE
Hence th e ch a ra cter decoder a c ts as an AND g a te .
The ou tp u ts o f th e s i x t y ch a r a cter decoder g a tes must be 'OR'ed t o
gether and then fe d to the i n t e n s i f i c a t i o n c i r c u i t r y . This would r e
q u ire a s i x t y in p u t 'OR' gate which o b v io u s ly exceeds the f a n -in cap
a b i l i t i e s o f DEC 2 MHa c i r c u i t s . N o tic e , however, th a t o n ly one char
a c te r decoder g ate can be a c t iv e a t one tim e ( i . e . t h e ir ou tp uts are
e x c l u s i v e ) . By sim p ly w ir in g -toe ou tp u ts o f th e s i x t y decoder g a te s
to g e th er w ith a s in g le clamped lo a d we are ab le to perform th e 'OR'
o p era tio n and. n o t u se any l o g i c a l hardware. In f a c t , we have fr e e d
59 clamped lo a d s which cou ld be u sed e lse w h e r e . F ig . 2 .8 g iv e s th e l o
g ic diagram o f th e ch a r a cter d ecoder.
TWO METHODS OF BEAM POSITIONING USING AN ANALOGUE HORIZONTAL SWEEP
3*1 In tro d u ctio n
In 1968* a p r o j e c t was begun by t h i s department which has now
brought us c lo s e to th e com pletion o f a complete grap hics te r m in a l.
This w i l l be d is c u s se d fu r th e r i n chapter VI.
O r ig in a lly i t was d ecid ed th a t a sim p le, lo w -c o s t ch a ra cter gener
a to r should be c o n str u c ted which used a r a s t e r sweep method. At th a t
tim e , the method o f unique p u lse g en era tio n which was d e scr ib e d i n chap
t e r I I was d evelop ed . A t e s t model was co n stru cted to dem onstrate the
f e a s i b i l i t y o f th e method. I t was th en decided to in co rp o ra te th e char
a c te r generator in t o a la r g e r , more s o p h is t ic a te d system .
During th e subsequent developm ent, th ree methods o f r a s t e r genera
t i o n have been t e s t e d . A ll u t i l i z e th e same method o f s e l e c t i n g the
ch aracter p u lse t r a in s d e scr ib e d i n th e preceding ch a p ter . Each method
arose as th e p r o je c t proceeded toward a goal o f in c r e a se d speed and f l e x
i b i l i t y .
The two methods o f beam p o s it io n in g which use a h o r iz o n ta l sweep
c i r c u i t w i l l be d e scr ib e d i n the remainder o f t h i s ch a p ter.
3 .2 H orizo n ta l and V e r t ic a l Sweep Method
8
In 1968-69, another M aster's c a n d ita te d esign ed and constructed,
a lim ited, t e s t model o f an A/N ch a ra cter generator which s u c c e s s f u lly
generated the l e t t e r s "A" and "C”.
26
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His method o f g en era tin g th e v e r t i c a l sweep was to u t i l i z e th e saw
to o th output o f a TEKTRONIX typ e 5^5 o s c illo s c o p e tr ig g e r e d by p u lse s l f
8 , 1 5 , 22 and. 29 which were d e r iv ed from th e cou nter d ecoder. This pro
duced f iv e lin e a r sweeps which were fe d to the Y -a x is o f a TEKTRONIX
typ e 56^ sto r a g e o s c i l l o s c o p e .
To provide a h o r iz o n ta l sweep, he used an in t e g r a te and. h o ld c i r
c u i t which charged a c a p a c ito r a t a uniform r a te during th e a c tu a l gen
e r a t io n o f th e c h a r a c te r . The f i n a l v o lta g e was h e ld u n t i l th e n e x t
ch a ra cter was generated and th e c a p a c ito r would, a g a in be charged to a
h ig h er v o lt a g e . The output o f t h i s in t e g r a to r was fe d to th e X -a x is o f
th e sto ra g e o s c il lo s c o p e .
When th e ou tp ut o f th e c h a r a cter decoder *0R' g a te was fe d to th e
Z -a x is o f th e sto r a g e o s c i l l o s c o p e , ch a ra cters cou ld be g en erated as
shown i n T ig . 3«1»
While t h i s method worked, i t was s e v e r e ly lim it e d i n speed by th e
in t e g r a te and h old c i r c u i t . Due to th e la r g e c a p a c ito r which was u sed ,
more than 100 msec were req u ired to d isch arge i t . This meant t h a t ,
w h ile a l i n e cou ld be w r itte n a t a f a i r l y h igh ch a ra cter sp eed , a f u l l
te n th o f a second was req u ired b efo re th e n ex t l i n e cou ld be w r it t e n .
For a low c o s t ch a ra cter gen erator u t i l i z i n g a sto r a g e c . r . t . th e
above r a s te r g en era tin g tech n iq u e would be s u f f i c i e n t . I f a normal
telep h o n e channel capable o f tr a n s m ittin g 1 ,2 0 0 b i t s / s e c . were used to
provide th e ASCII codes to th e ch a ra cter g en era to r, t h i s method would
be i d e a l .
Of co u rse, much f a s t e r sweep c i r c u i t s can be c o n stru cted as can
in t e g r a t e and h old c i r c u i t s . S in ce th e v e r t i c a l sweep c i r c u i t must on
l y sweep o u t a s in g le ch a ra cter h e ig h t , i t could be r e l a t i v e l y n o n -lin e a r
BUFFER
PULSE INHIBIT
GATE
Z-Axis SPECIAL
CHARACTER
DECODER CHARACTER
DECODER
PULSE COUNTER
C£
GATES
COUNTER
DECODER
o\
pH CO rH - - .
—«,i —m —i» —u - t »
=;^5: =f?=:
HORIZONTAL
:ES; SWEEP
X-Axis
VERTICAL
RESET* SUMMER
SWEEP
Y-Axis d/a
3 BIT DOWN COUNTER COUNT*
Figure 3*2 C haracter Generator With H o rizo n ta l and V e r tic a l Sweep
